R-f window for high power electron tubes



May 17, 1966 D. H. PREIST ETAL R-F WINDOW FOR HIGH POWER ELECTRON TUBES Filed April 16, 1962 UNCOATED-V, \Q-conran DONALD H. PRE/ST RUTH CARLSON TALCOTT Y WX EMZ ATTORNEY 3,252,034 R-F WINDOW FOR HIGH POWER ELECTRON TUBES Donald H. Preist, Mill Valley, and Ruth Carlson Talcott, Berkeley, Calif., assignors to Eitel-McCullough, Inc., San Carlos, Calif., a corporation of California Filed Apr. 16, 1962, Ser. No. 187,521 12 Claims. (Cl. 313-107) This invention relates to windows transparent to radiofrequency electromagnetic energy, and particularly to such windows fabricated in a manner to render them less susceptible to heating due to electron bombardment.

In the electron tube industry, one of the limiting factors determining the amount of power which can be extracted from an electron tube involves the output means for the tube. The output means is often in the form of dielectric material forming a window transparent to radio-frequency electromagnetic energy. The limiting factor is that the amount of power which a window is capable of passing has not kept pace with the amount of power which an electron tube is capable of generating. Thus, while it is possible with state-of-the-art electron tubes to generate very high powers, it is becoming increasingly difficult to extract such power from the electron tube because of the limitations imposed by the output means, especially where such output means is a dielectric window as used in microwave tubes such as. klystrons, for instance.

Output \m'ndows transparent to electromagnetic energy are usually fabricated from dielectric materials having. a coeflicient of secondary electron emission greater than unity for values of an accelerating electric field greater than about 40-80 volts, and this characteristic renders them susceptible to destructive heating due to single surface multipactoring by secondary electrons emitted from the window itself when the accelerating electric field exceeds a critical value. This phenomenon is explained at length in an article by the inventors herein, appearing in the IRE Transactions of the Professional Group on Elec tron Devices, volume ED-S, Number 4, dated July 1961. It is therefore one of the objects of the invention to provide a composition of dielectric material which possesses a maximum coefficient of secondary emission of electrons of about unity or less for values of bombarding electron velocity at least several times higher than the usual range of 40-80 volts.

Another object of the invention is to provide an article of manufacture fabricated from a dielectric material hav-,

ing a maximum coeflicient of secondary electron emission of not substantially more than unity under the conditions stated in the preceding paragraph.

Another of the important objects of the invention is to provide a radio-frequency output window fabricated from a material having a secondary electron emission coefficient of about or less than unity in the presence of free electrons in vacuum and bombarding electron velocity substantially higher than 4080 volts.

Still another object of the invention is the provision of an output window having a coeflicient of secondary electron emission of about or less than unity and which also possesses thermal stress resistance sufiicient to withstand temperature gradients caused by dielectric losses, for example, without failure due to fracture, softening, or chemical instability usually associated with such temperature gradients.

Still another important object of the invention is the provision of a radio-frequency output window having a coefficient of secondary electron emission of about or less than unity, and which also possesses the requisite dielectric strength even at elevated temperatures to withstand very high voltage gradients.

United States Patent A still further object of the invention is the provision of a high power radio-frequency output window for electron tubes which possesses chemical stability even when subjected to extremes in temperature such as during bakeout.

Still another object of the invention is the provision of a microwave structure incorporating such a window.

A still further object of the invention is the provision of a dielectric and metallic microwave structure incorporating a dielectric radio-frequency window fabricated from material treated to possess a secondary electron emission coefficient of about or less than unity in vacuum and in the presence of free electrons, and in which the metallic parts of thestructure have also been treated so as to have a secondary electron emission coeflicient of about or less than unity.

A still further object of the invention is to provide an electron tube having at least one resonant cavity portion equipped with at least one radio-frequency window which possesses a secondary electron emission coefficient of about or less than unity, and in which the conductive metallic walls of the resonant cavity also possess a secondary electron emission coefficient of about or less than unity.

The power-passing limitations which afflict radiofrequency windows are applicable alike to cylindrical and fiat resonant cavity windows and also to flat. and conical windows such as are used in waveguides. -It is therefore another object of the invention to provide a microwave window assembly constructed in a manner to decrease electron bombardment of the window and thereby decrease the amount of heat generated in the window, and to dissipate by efficient conduction such heat as is generated in the window.

A still further object of the invention is the provision of a resonant cavity portion for a klystron tube in which which reduces to about or less than unity the coefiicient' of secondary electron emission from both the dielectric and metal parts.

The invention possesses other objects and features of value, some of which, with the foregoing, will be apparent from the following description and the drawings. It is to be understood, however, that the invention is not limited to the embodiments illustrated and described, but may be incorporated in varying forms within the scope of the appended claims.

Broadly considered, the invention in one of its aspects comprises the provision of a dielectric material having a coefficient of secondary electron emission of about or less than unity for values of bombarding electron velocity at least several times higher than the usual range of 40 volts; and in another aspect the utilization of such material in combination with metallic bodies or surfaces which also have a secondary electron emission coefiicient of about or less than unity to provide radio-frequency' high-power output Windows for microwave devices. It

is well known in the art that dielectric materials suitable over a wider range of bombarding velocities by the application thereto of titanium by a method described and claimed in a copending application. The titanium is preferably incorporated in the dielectric material after the dielectric material has been fabricated into the desired configuration for use as an output window, but it is believed that such incorporation may also be effected during the manufacture of the dielectric material. In the former case, it has been found that titanium deposited on selected dielectric materials by vacuum deposition to a thickness of about 100 Angrostrom units or less will result in a dielectric composition which retains the requisite dielectric strength to be used as a high-frequency, highpower output window, together with the other requisite characteristics pertaining to dielectric loss, thermal stress, and chemical stability, as discussed above. When incorporated in a metallic structure such as a resonant cavity or waveguide, the metallic structure is also provided with a layer of titanium, so that the secondary electron emission coefficient of the metallic surfaces will also be about or less than unity. Such a window construction has been found to be completely free from destructive heating due to secondary emission of electrons from the surface of the window itself, or from secondarily emitted electrons originating in the adjacent metallic structure.

Referring to the drawings:

FIGURE 1 is an elevation of a klystron tube incorporating a radio-frequency output window in accordance with the invention. The output window assembly is shown in vertical section.

FIGURE 2 is an enlarged vertical sectional view of that portion of FIGURE 1 indicated by the bracket 2. The titanium coatings on the interior surfaces are shown in exaggerated thickness for clarity.

FIGURE 3 is a vertical sectional view through a waveguide portion equipped with a window according to this invention.

FIGURE 4 is a graph showing the secondary emission coeflicient (6) as a function of velocity (V) of bombarding electrons expressed in volts for coated and uncoated dielectric windows. 7

In the early days of UHF, when klystron tubes, for instance, were first coming into their own, radio-frequency transparent windows were almost exclusively made of glass, and were capable of handling only a few kilowatts of CW power. In such early klystron tubes, power output was limited by failure of the window in the output cavity, and such failure was ascribed to overheating of the glass window caused by dielectric losses in the window material. As the industry became more knowledgeable, ceramic materials having greater resistance to thermal stresses were substituted forglass materials. The geometries or configuration of associated metallic structures also changed in order to take advantage of ceramic ma: terials. Ceramic windows enable reliable operation at the lO-kilowatt level at frequencies between 700 and 1,000 megacycles, and considerablyhigher power at lower frequencies. In the race to develop tubes of the klystron type having higher frequencies and higher output power, the assignee of the present invention pioneered the utilization of external resonant cavities coupled to an electromagnetic field within the klystron envelope through a dielectric or ceramic Window. A klystron tube of this type is illustrated, described and claimed in U.-S. Patent No. 2,619,611, issued to the assignee of the present invention. As the power output increased from these new and better tubes, the limiting factor again became the tendency of the large output windows to rupture, thus destroying the envelope. Even integral cavity tubes, which utilize dielectric or ceramic windows interposed in a waveguide output, suffer from the inability of passing through the waveguide window the large power output from these tubes.

-It was believed at the outset that the dielectric losses of the dielectric material itself, usually ceramic, caused heating of the ceramic material which resulted in rupture thereof. Such dielectric losses do of course cause heating, but usually not suificient heating to rupture the window. Another theory that has been advanced to explain the rupturing of windows at high power units is that electrons from the electron beam or other source bombard the window and cause rupture. A still further theory is that secondary electrons, released from the metallic output gap drift tube assemblies by bombarding primaries of the beam, arrive at the window with high velocity, the kinetic energy of the electrons being transformed into heat on the window, and'the window being caused to rupture as a result thereof. It has been found that under some special circumstances excessive multipactoring at the output gap can cause such bombardment of the window, but in the usual case does not occur.

Through an extensive study program it has been determined that in the usual case heating and result-ant rupture of dielectric output windows is caused by a phenomenon known as single surface multipactoring on the surface of the dielectric material itself, trigg red by an electron or electrons arriving at the window from within the envelope and impinging on the window with a force at least sufficient to liberate secondary electrons from the dielectric material. The phenomenon is called two-surface multi pactoring when the multipactoring occurs between a waveguide surface and the adjacent surface of an inclined waveguide window, or between the diverging surfaces adjacent the apex end of a conical waveguide Window. The triggering electrons do not usually come from the beam, but rather are emitted from adjacent metal parts as a result of the presence of high intensity electric fields. It has been found that free secondary electrons liberated from the dielectric window material are accelerated to critical velocities by the alternating electric field adjacent the vacuum side of the window and are redirected toward the dielectric window such that additional free secondaries are liberated from the dielectric material when the accelerated and redirected free secondary electrons impinge on the dielectric window. As illustrated by the full line in the graph of FIGURE 4, for conventional uncoated or untreated radio-frequency windows, the critical field strength required to accelerate an electron sufficiently to commence single surface mul-tipactoring and heating as a result thereof corresponds to bombarding electron velocities ranging between 40 and volts.

After considerable experimentation and analysis, it has been determined that by providing a coating-or layer of some material on the inside surface of the dielectric material and adjacent metallic parts, composed of a material having a low secondary emission characteristic, beneficial results can be obtained in that the value of the critical field strength required to accelerate electrons to a bombarding velocity sufficient to commence single surface multipactoring is greatly increased as shown by the dash line in the graph of FIGURE 4. At this higher critical field strength electrons starting from rest are accelerated in (2n+1) half cycles to a bombarding velocity more than suflicient to release one secondary electron per bombarding electron (corresponding to a coefficient of secondary electron emission of unity) on impact with a conventional window. As seen from the graph, the critical field strength has been increased so that it approaches the critical field strength at which maximum secondary emission occurs, with maximum secondary emission occurring at about or not substantially above unity, as shown. The realization of these beneficial results depended on the determination of a material to put on the dielectric and metal surfaces which would not have deleterious effects either on the electromagnetic transparency of the window or its dielectric loss, its chemical stability, its resistance to thermal stress, and which would not have the tendency of forming an oxide coating on interior metallic surfaces. Other important factors also had to be considered, such as the ability of the coating to withstand bakeout temperatures and to be uncontaminating to other associated parts. A

In addition to the problem of determining what coating material to use, a secondary problem but of equal importance was the problem of determining the thickness of the coating or film and the method of its application. To be effective as an electromagnetic window, the dielectric material obviously must remain transparent to electromagnetic energy, and the interior surface of thewindow must therefore be nonconductive, or if conductive in some small degree, the dielectric loss of the material must remain within certain limits. Another problem was the method of controlling the thickness of the coating material so that the effects would be reproducible from one body to another, and from one electron tube to another. The method of application of such a coating, and the method of determining the thickness, has been described in a copending application- It has been found that a body of dielectric material normally having in the presence of free electrons and a radio-frequency alternating electric field of sufficient intensity to accelerate the free electrons to a critical bombardment velocity where the coefficient of secondary emission of such electrons is greater than unity, may be transformed into a body of dielectric material having a coefiicient of secondary emission of about or less than unity for a greatly expanded range of electric field intensities or bombarding velocities by application to one or all of its surfaces of an extremely thin coating or layer of titanium. This transformation is illustrated graphically in FIGURE 4 which at V indicates the usual critical field strength for conventional uncoated windows, and at V indicates the expanded range of field strength required before a critical value of bombardment velocity is reached. Titanium may be applied as a coating or layer on the window after its manufacture and formation into the requisite configuration, and it is believed the titanium may be incorporated in a surface zone of the dielectric material during the process of manufacture of specific window configurations from the dielectric material. In either case it is de sirable that the coating or layer of titanium be of reproducible thickness, and that the electrical resistance of the dielectric material remain in the range between to 10 ohms per square. It has been found that in order to maintain resistance of a dielectric body within the requisite range, a film is required which is so thin as to be invisible either on an opaque dielectric surface or on a transparent dielectric surface.

The secondary electron emission yield of an electrically conducting surface is determined by the composition of its surface to the depth of penetration of a bombarding primary electron. In a radio-frequency (R-F) device, the penetration depth is not very great since the velocity of electrons traveling in an R-F field is limited to that velocity which can be attained in one-half of one R-F cycle. In many practical devices, this does not exceed a few hundred volts at a dielectric surface and a few thousand volts in the higher voltage regions of a klystron or traveling wave tube, for instance. Accordingly, it has been found that films on the order of 1,000 Angstrom units of a dense material such as titanium are suflicient to diminish to about or less than unity the secondary emission yield in the highest voltage regions of an electrically conducting surface, and that films 100 Angstrom units or less are adequate on dielectric surfaces to diminish the secondary emission yield to about or less than unity. In the case of the dielectric surface, however, the coating must be discontinuous in nature in order that the electrical resistance remain between 10 and 10 ohms per square. Such a discontinuous nature in the coating provides the high resistance while also providing the thickness required to control the secondary electron emitting characteristics of the surface.

In FIGURE 2 is illustrated the internal resonant cavity portion 2 from the external resonant cavity type klystron illustrated in FIGURE 1, which includes an electron gun section 4, a radio-frequency interaction section 6, and a collector section 7. As is well known in the art, the electron gun section, the radio-frequency section, and the collector section are hermetically united in axial alignment to enable the projection of an electron beam through a series of drift tube sections 8, each terminating within a cavity in a conically tapered end portion 9 spaced from the associated end of an adjacent drift tube section to provide an interaction gap 12 therebetween. Drift tube sections 8 are supported in axially spaced alignment by relatively heavy transversely extending annular metallic plates 13, preferably fabricated from oxygen-free highconductivity copper. These plates, in turn, are held in axially spaced relationship by the cylindrical dielectric radio-frequency windows 14, one of which is designated as the output window. The opposite ends of the cylindrical dielectric windows are hermetically united to the associated drift tube supporting plates by sealing flange structures 16 described at greater length in United States Patent 2,903,614. Such a sealing structure introduces flexibility in the window design to accommodate differences in expansion and contraction of the dielectric window and the metallic drift tube assembly.

As illustrated in FIGURE 2, the inner surface 17 of the cylindrical dielectric window is provided with a thin coating or layer 18 of titanium, deposited thereon by the method described and claimed in a copending application. The coating is preferably deposited by vapor deposition of pure titanium metal in vacuum in a manner to provide a uniformly thick coating of titanium over the inner surface of the dielectric window. To complete the cavity, the adjacent metallic surfaces 19 of the drift tube supporting plates, the conical metallic surfaces 21 of the drift tube tips within the cavity, and a portion of the surface 22 forming the inner bore of each drift tube tip, are coated with a layer 23 of pure titanium metal to a thickness of about one-tenth of a mil. In FIGURE 2 the layer of titanium on these surfaces is exaggerated in thickness for clarity.

Experience has taught that a layer of titanium on these surfaces somewhat less than one-tenth of a mil thick is satisfactory and will reduce the secondary electron emission coefiicient of these metal parts to a value of about or less than unity for values of electric fields at which other characteristics, such as external arc-over, become limiting factors. Because coatings of such scant thicknesses are difficult to measure, it has been found that when the characteristically copper-colored parts are completely opaque to visual inspection due to the deposition thereon of pure titanium, the coated metallic parts will appear grey and the thickness of the titanium will be adequate to provide the requisite low secondary electron emission characteristic. It will of course be understood that where desirable, any or all windows of the klystron, in addition to other dielectric bodies, such as the electron gun ceramics, maybe coated with titanium with beneficial results. It should also be understood that the metallic surfaces within the evacuated envelope of an integral cavity klystron, including a waveguide output having a flat output 'window, may be coated as described with beneficial results.

In FIGURE 3 is illustrated a flat circular dielectric Waveguide window 26 having its periphery 27 hermetically united to the inner surface 28 of a metallic waveguide 29. One surface of the window is provided with a layer 31 of titanium about Angstrom units thick, while the adjacent inner metallic surface of the waveguide is provided with a coating 32 of titanium at least 1,000 Angstrom units thick, or in the alternative, about onetenth of a mil.

Regarding choice of materials, it is preferred that the metallic elements of the combination be fabricated from oxygen-free, high-conductivity copper, and that the dielectric materials be selected from the group including fused silica, alumina-silicate glass, steatite, fosterite, alumina, beryllia or pyroceram. Materials selected from this group and supporting plate will be found to have a dielectric constant ranging from about 3.8 for fused silica to about 8 or 9 for alumina. In terms of the power factor or loss tangent, it will be found that a selection may be made from this group in which the value of this characteristic is in the range between .0001 for fused silica to about .0008 for beryllia for frequencies of about 10 cps. The thermal coefficients of expansion of these materials range between about 10 for fused silica to about 110x10 for forsterite. The modulu'ses of elasticity of the selected materials range between-a low of 10 for fused silica to a high of about 40 10 for alumina. Of these six materials, the three that are preferred for microwave applications are fused silica, alumina and beryllia.

In conclusion, it may be stated that any radio-frequency window of any configuration will be heated to some extent by various causes.- It is important to conduct away as much of such heat as possible, regardless of the cause. It has been found that windows fabricated from beryllia, coated with titanium, and incorporated in metallic structures fabricated from oxygen-free, high-conductivity copper which are also coated with titanium provide an almost ideal window package.

We claim:

1. As an article of manufacture, a stable body of dielectric material at least one surface of which incorporates titanium and which in the presence of free electrons in vacuum and a high radio frequency electric field possesses a coefficient of secondary emission having a maximum value'of substantially unity and an electrical resistance of between about 10 and 10 ohms per square.

2. As an article of manufacture, a body of low loss dielectric material having a maximum coeflicient of secondary electron emission greater than one, said body having a surface thereon which incorporates titanium and possesses a maximum coefficient of secondary electron emission substantially equal to one and an electrical resistance of between about 10' and about 10 ohms per square.

3. An article of manufacture comprising a substrate of low loss dielectric material and a coating of titanium on a surface of said substrate, the maximum value of the coefficient of secondary electron emission of said coated substrate being substantially unity and the electrical surface resistance of said coated substrate being between about 10 and about 10 ohms per square.

4. An article of manufacture as claimed in claim 3 wherein said coating is discontinuous.

5. An article as claimed in claim 3, in which said dielectric material is selected from 'the group consisting of alumina, alumina-silicate glass, beryllia, fused silica, steatite, forsterite and pyroceram, and said coating comprises titanium deposited thereon.

6. An article of manufacture comprising a substrate of dielectric material and a coating consisting essentially of titanium on a surface of said substrate, said coating having a thickness averaging about 100 Angstrom units and a coeflicient of secondary electron emission the maximum value of which is substantially unity.

7. An article as claimed in claim 6 wherein said coating is discontinuous.

8. An article as claimed in claim 6 wherein said dielectric material is selected from the group consisting of alumina, alumina-silica glass, beryllia, fused silica, steatite, forsterite and pyroceram, and said coating comprises titanium deposited thereon.

9. In an evacuated device including an electrically conductive metallic portion and an electrically nonconductive-dielectric portion each having a surface exposed to vacuum within the device, a coating of titanium on the exposed surfaces of said conducting and nonconducting portions exposed to vacuum to suppress the secondary emission of electrons therefrom, the coating on said nonconducting portion of said evacuated device having an electrical surface resistance of between about 10' and about 10 ohms per square.

10. In an evacuated device including an electrically conductive metallic portion and an'electrically nonconductive dielectric portion each having a surface exposed to vacuum within said device, a coating of titanium on the surface of said nonconducting portion exposed to vacuum to suppress the secondary emission of electrons therefrom, said coated surface of said nonconducting portion having an electrical resistance of between about 10 and about 10 ohms per square.

.11. An evacuated device according to claim 10 in which said coating is discontinuous.

12. An evacuated device according to claim 10 in which said nonconducting dielectric portion is selected from the group of dielectrics consisting of alumina, alumina-silicate glass, beryllia, fused silica, steatite, fors-terite and pyroceram.

References Cited by the Examiner UNITED STATES PATENTS 2,613,304 10/1952 Colinet 252112 2,754,448 7/1956 Van Aperen 315-539 2,770,784 11/1956 Hatch 33398 2,855,491 10/1958 Navias 252112 2,924,540 2/ 1960 DAndrea 117227 2,929,035 3/1960 Winslow et a1. 333-98 2,955,229 10/1960 Bondley 315--5.39 2,971,172 2/1961 Hamilton et a1; 333--98 2,981,908 4/1961 Thompson et al 333-98 2,990,526 6/ 1961 Shelton 33398 3,030,237 4/1962 Price 117227 FOREIGN PATENTS 1,295,748 5/ 1962 France.

OTHER REFERENCES Hayes et al.: Research on Microwave Window Multipactor and Its Inhibition, Eitel-McCullough Inc., San Carlos, Calif, pp. 14 and 15 relied on.

RICHARD D. NEVIUS, Primary Examiner.

JOSEPH SPENCER, Examiner.

C. O. GARDNER, W. L. JARVIS, Assistant Examiners. 

10. IN AN EVACUATED DEVICE INCLUDING AN ELECTRICALLY CONDUCTIVE METALLIC PORTION AND AN ELECTRICALLY NONCONDUCTIVE DIELECTRIC PORTION EACH HAVING A SURFACE EXPOSED TO VACUUM WITHIN SAID DEVICE, A COATING OF TITANIUM ON THE SURFACE OF SAID NONCONDUCTING PORTION EXPOSED TO VACUUM TO SUPPRESS THE SECONDARY EMISSION OF ELECTRONS THEREFROM, SAID COATED SURFACE OF SAID NONCONDUCTING PORTION HAVING AN ELECTRICAL RESISTANCE OF BETWEEN ABOUT 107 AND ABOUT 1011 OHMS PER SQUARE. 